Literal translation
the chestnut smokes, is about to burst
Meaning
tension is rising, it's about to blow up
Explanation
Castagno means chestnut, but also, in familiar extension, a brawl or a punch, tubo means mist or vapor, and peta means to burst, to explode, or to pop; la castagno tubo vai peta thus literally translates as the chestnut smokes or is about to burst, the image resting on a chestnut left too close to the fire that starts smoking before exploding under heat pressure. In use, the expression signals that tension is rising, that things are about to blow up, the exact Provençal equivalent of the French there's a storm brewing applied to human relations about to degenerate into a quarrel or violence. The double meaning of castagno, both the fruit and a brawl, strengthens the image's relevance: the chestnut about to pop literally prefigures the castagne, the brawl, about to break out. The register is familiar and vivid, used to warn of an overheated atmosphere before open conflict breaks out among angry people.